The present invention relates to a method and apparatus for detecting a power-on condition in an electronic circuit. In particular, the present invention relates to electronic power-on sense circuits that operate under low power and low current conditions.
Various power-on detector circuits are used in electronic systems to ensure that a predictable power-up condition can be achieved. In digital circuits, it is possible that a digital logic circuit may find a trapped state or other undetermined condition during power up. Similarly, in analog circuits, a particular circuit may find a condition where transistors lock themselves in an off condition during power. Start-up and logic initialization problems may be avoided by incorporating circuitry into the electronic system that initializes each of the various circuits to a known condition. A predictable power-on reset circuit may be employed to detect the power-on condition and appropriately initialize the various circuits during a power-on sequence.
Briefly stated, in accordance with the present invention a power-on sense circuit accurately senses a power-on condition when a power supply voltage exceeds a desired trigger voltage level. The power-on sense circuit includes a voltage-to-current converter circuit and a beta-multiplier reference circuit The voltage-to-current converter circuit and the beta-multiplier reference circuit generate currents that relate to the power supply voltage. By sensing a balanced current operating condition with the beta-multiplier reference circuit, the power-on sense circuit determines when a desired trigger voltage has been achieved. The trigger voltage level has a zero temperature coefficient at median operating temperatures, and has a slightly downward curvature shape without the need for high-current resistor-dividers or bandgap circuits. The power-on sense circuit may be adapted for use as a power-on reset signal. By adding an amplifier stage to the output signal, the power-on sense circuit may also be used as an analog reference voltage generator. The design of the power-on sense circuit is scalable and maybe used under low-voltage and low-current operating conditions.
According to a feature of the invention, an apparatus for sensing a power-on condition in an electronic system having a power supply with an associated power supply voltage, includes: a first circuit that produces a first current in response to the power supply voltage, a second circuit that produces a second current that is a beta multiple of the first current, and a third circuit that produces an output signal in response to comparing the first current to the second current such that the output signal indicates the power-on condition when the power supply voltage reaches a trigger voltage. The first, second and third circuits may be combined into a single circuit. The first circuit may include a transistor having a gate connected to the power supply voltage such that the transistor conducts the first current by converting the power supply voltage to the first current. Similarly, the gate connection of the transistor may be coupled to another voltage that is related to the power supply voltage such that another current is produced by the transistor responsive to the power supply voltage. The first circuit may also include a current mirror circuit that produces the first current in response to another current that is related to the power supply voltage.
According to a further feature of the invention, the source of the transistor in the first circuit is coupled to a circuit ground through a fourth circuit. The fourth circuit may include at least one of a resistor, a transistor, and a diode. The fourth circuit is arranged to change the trigger voltage by changing the voltage that appears at a source terminal of the transistor relative to the transistors gate voltage.
According to another feature of the invention, the output signal is generated when currents are balanced in the first and second circuits. The second circuit is a beta-multiplier circuit. The second circuit includes a first transistor and a second transistor that share a common gate connection, wherein a ratio is associated with the beta of the first transistor and the beta of the second transistor, and the ratio is different from unity such that the first transistor and the second transistor have balanced currents at the trigger voltage. The first and second transistors are arranged to operate in weak inversion when the power supply voltage is at the trigger voltage. Also, the second circuit includes a resistor that is coupled between the source of the second transistor and a circuit ground. The first and second transistors operate at a bias point between square-law operation and exponential operation when the power supply voltage is at the trigger voltage. The output signal has a substantially zero temperature coefficient for median temperatures. An amplifier circuit may be adapted to produce an analog reference signal in response to the output signal. Also, an inverter circuit may be employed to produce a digital signal in response to the output signal. A capacitor may be coupled to the apparatus and arranged to couple the power supply voltage to at least one of the first, second, and third circuits such that the output signal is initialized to an operating state and changes in the power supply voltage require a transition time before the output signal changes to another operating state.
According to another feature of the invention, a method of sensing a power-on condition in an electronic system, includes: converting a power supply voltage to a sense current that is related to the power supply voltage, mirroring the sense current to produce a second current, generating a reference current that is a beta multiple of the second current, comparing the reference current to the second current, and producing an output signal in response to comparing the reference current to the second current such that the output signal indicates a first operating state when the power supply voltage is a first voltage, and a second operating state when the power supply voltage is a second voltage different from the first voltage whereby the power-on condition is sensed when the output signal indicates the second operating state. Generating the reference current includes conducting the second current with a first transistor and conducting the reference current with a second transistor that is a beta multiple of the first transistor.
According to still another feature of the invention, an apparatus for sensing a power-on condition, includes: a means for producing a first current related to a power supply voltage, a means for producing a second current that is a beta multiple of the first current, a means for producing an output signal that is responsive to a comparison between the first current and the second current such that the output signal indicates a power-on condition when the power supply voltage is at a trigger voltage. The apparatus may further include means for generating one of a digital signal and an analog reference voltage from the output signal.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.